1. Field of the Invention
The present invention relates to stretched products comprising a propylene-based resin composition excellent in cold-stretchability and which have a low degree of dimensional nonuniformity and a high low-temperature shrinkability, and a process for producing the stretched products.
2. Related Art
Currently, in the case of shrinkable plastics, a stretched product which shrinks at a low temperature is desirable from the viewpoint of energy savings, improvement of safety in work environments, etc. In the present invention, the term "stretched products" is used to denote products including not only flat films and sheets obtained by stretching but also various products of a non-flat shape in a stretched state obtained by molding or the like. In particular, heat-shrinkable films and sheets are advantageous in that they prevent their contents from being changed in quality by heat, and hence there has been a growing demand for such materials which shrink at low temperatures. In shrink packaging, as the shrinkage percentage is increased, the temperature at which sufficient shrinkage can be attained is lowered, and a wrinkle-free beautiful package can be obtained more easily.
In the field of packaging films and sheets, because of the demand for higher-speed automatic packaging, packaging films and sheets each having a uniform thickness and a good flatness are required. Also, in the case of printing machines, bag making machines- and molding machines, there is a growing demand for improved efficiency. Particularly in the field of films, rapid and secure sealing by means of a high-speed packaging machine is required, and therefore nonuniformity in thickness of a film or sheet itself becomes a cause of the nonuniformity of sealing capability. When a film or sheet having an insufficient flatness is sent out at a high speed during printing or bag making, bending and sagging occur, so that high-speed printing or bag making cannot be achieved.
Propylene-based resins have a relatively high melting point, a high heat resistance and a high oil resistance and hence are desirable as materials for resin compositions for packaging of oil-containing substances, which compositions are to be heated.
On the other hand, since ethylene-based resins such as ethylene-vinyl acetate copolymers have a low melting point, they permit lowering of the stretching temperature and can give a stretched product having a good low-temperature shrinkability. These resins, however, are poor in heat resistance and oil resistance. That is, packaging materials obtained from them cannot withstand heat sterilization at 90.degree. C. or higher and cannot withstand cooking of an oil-containing substance packaged using the packaging materials.
A technique of stretching a propylene-based resin at a high temperature near its melting point to cause orientation is well known. As packaging films having a propylene-based resin layer, there are known monolayer films comprising a mixed composition of propylene-based resins and various composite laminated films. Also well known are films of compositions obtained by mixing a propylene-based resin with a tackifier. However, although the compositions used therein have a high heat resistance because of their melting point being higher than that of ethylene-based resins, they are poor in cold-stretchability and hence require stretching at a high temperature of 100.degree. C. or higher.
Also well known are oriented films of compositions obtained by mixing a propylene-based resin with a polybutene resin, but the cold-stretchability of the compositions used therein is not sufficient and the oriented films do not have a thickness precision sufficient for their use in a present-day high-speed machine and moreover are not satisfactory in low-temperature shrinkability.
On the other hand, it has also been attempted to laminate a proplene-based resin layer on a resin having an excellent cold-stretchability and cold-stretch them together.
For example, Jap. Pat. Pub. No. 48-21354 discloses a monolayer film obtained by stretching a mixed composition of a propylene-based resin, a petroleum resin and an anti-blocking agent at 135.degree. to 160.degree. C.
Jap. Pat. Appln. Kokai (Laid-Open) No. 47-34565 (corresponding to U.S. Pat. No. 3,741,253) discloses a laminated film obtained by laminating a vinylidene-chloride-based resin layer on a crosslinked EVA base layer (an inner seal layer), further laminating thereon a layer (an outer surface layer) of a mixed composition of a polypropylene resin, a polybutene-1 resin and an atactic polypropylene resin, and stretching the three layers with heating at about 88.degree. C.
The specification of U.S. Pat. No. 3,808,304 discloses a film obtained by stretching a monolayer film of a mixed composition of a polypropylene resin and a polybutene-1 resin with heating at approximately 82.degree.-110.degree. C.
The specification of U.S. Pat. No. 3,832,270 discloses a laminated film obtained by stretching with heating at 98.degree.-100.degree. C. and which comprises three layers, i.e., a crosslinked EVA base layer (an inner seal layer) and two layers (outer surface layers) of a mixed composition of a polypropylene resin, a polybutene-1 resin and an atactic polypropylene resin.
Jap. Pat. Appln. Kokai (Laid-Open) No. 54-92895 discloses a laminated film obtained by stretching with heating at 130.degree. C. and which comprises two layers, i.e., a base layer formed of a mixture of a polypropylene resin and a low-molecular-weight resin (e.g., a petroleum resin) and a layer of a mixed composition of a polypropylene resin and a polybutene-1 resin.
Jap. Pat. Appln. Kokai (Laid-Open) No. 54-106585 (corresponding to of U.S. Pat. No. 4,230,767) discloses a laminated film obtained by stretching with heating at 130.degree. C. and which comprises three layers, i.e., a base layer of a resin composition composed mainly of a polypropylene type polymer and two surface layers of a mixed composition of an ethylene-propylene copolymer, a butene-1-based copolymer and a low-molecular-weight resin(a low-molecular-weight polypropylene or a low-molecular-weight polyethylene).
Jap. Pat. Appln. Kokai (Laid-Open) No. 57-15958 discloses a laminated film obtained by stretching with heating at 70.degree.-130.degree. C. and which comprises three layers, i.e., a base layer of a mixture of a propylene-based polymer and a butene-1-based polymer and two surface layers of an olefin resin (e.g. a propylene-butene-1 copolymer) gaving a melting point of 150.degree. C. or lower.
Jap. Pat. Appln. Kokai (Laid-Open) No. 58-11147 (corresponding to U.S. Pat. No. 4,391,862) discloses a laminated film obtained by stretching with heating at about 88.degree. C. and which comprises four layers, i.e., a crosslinked EVA base layer, a barrier layer and two outer layers of a mixed composition of a polypropylene resin and a polybutene-1 resin.
Jap. Pat. Appln. Kokai (Laid-Open) No. 60-79932 (corresponding to U.S. Pat. No. 4,619,859) discloses a laminated film obtained by stretching with heating at 45.degree. C. and which comprises five layers, namely, a core layer of a mixed composition of a polypropylene resin and a polybutene-1 resin, intermediate layers as base layers on both sides thereof which are formed of a mixed composition very excellent in cold-stretchability composed mainly of an ethylene-vinyl acetate copolymer, and two surface layers formed of an ethylene-vinyl acetate copolymer.
However, the propylene-based resin compositions described in these references can give stretched products good in thickness uniformity only on stretching at a high temperature of about 100.degree. C. or higher and permit cold stretching only when they are laminated on a base layer formed of another composition excellent in cold-stretchability. Thus, the propylene-based resin compositions themselves do not have a satisfactory cold-stretchability.
That is, the propylene-based resin compositions poor in cold-stretchability are insufficient in low-temperature shrinkability, and products obtained by forming the propylene-based resin composition into a laminate and cold-stretching the laminate, have not been sufficient in thickness uniformity and do not have desired performance characteristics. In other words, for satisfactory cold stretching of the laminate, the thickness of the propylene-based resin poor in stretchability should be reduced, and therefore a desired heat resistance of the whole laminate film has been unattainable. For obtaining a film in which all layers constituting the multiple layers of the film have been stretched to a high degree, the propylene-based resin compositions have been disadvantageous, for example, as follows. Since different stretching conditions should be employed for different resins constituting the layers, undesirable phenomena such as thickness non-uniformity, streaks, punctures and whitening occur during stretching, and the resulting film has an insufficient low-temperature shrinkage percentage. Moreover, when the film is subjected to free shrinkage at a high temperature at which sufficient shrinkage can be attained, a propylene-based resin layer having a small shrinkage percentage becomes wavy and the film after shrinking shows whitening.
In addition, when a film having a high degree of thickness nonuniformity obtained by forced cold stretching of the propylene-based resin is wound round a bobbin, the wound film is uneven, and the film becomes insufficient in flatness, is poor in machinability in a sending-out step in bag making, printing or the like, and produces nonuniform sealing strength in a high-speed automatic packaging machine or the like.